In a small saucepan, bring water and vinegar just to a boil on the stove.
3/4 cup water, 3/4 cup white wine vinegar
Stir in sugar and kosher salt until dissolved. Remove from heat and let cool slightly.
1/4 cup granulated sugar, 1/2 tbsp. kosher salt
Add thinly sliced red onions and red pepper flakes to the mason jar and pour vinegar mixture over the onions and into the jar.
1 large (7-8 oz.) red onion, thinly sliced, 1 tsp. red pepper flakes
Cover with the lid and place into the refrigerator for at least 2 hours (preferably longer, overnight is best) to marinate and soften.
Notes
For optimal outcomes when making these pickled red onions, I recommended referring to the recipe details within the post. The recipe post commonly includes valuable notes, expert tips and frequently asked questions, that are not presented in the recipe card, often including how to make this recipe dairy-free, gluten free or vegan and ideas for healthy ingredient substitutions or variations.The provided nutrition information is merely an approximation, as variations can arise depending on the specific ingredients/brands you select.